Abstract [en]

Changes in vegetation are known to have an impact on climate via biogeophysical effects such as changes in albedo and heat fluxes. The magnitude and extent of these effects is however uncertain. Here the effects of maximum afforestation and deforestation are studied over Europe. This is done by comparing three regional climate model simulations: one with present day vegetation, one with maximum afforestation and one with maximum deforestation. In general afforestation leads to more evapotranspiration (ET) which leads to decreased temperature, while deforestation leads to less ET which leads to increased temperature. There are exceptions, mainly in regions with little water available for evapotranspiration. In such regions ET will not change even if vegetation changes. In such regions changes in albedo are relatively more important for temperature. The biogeophysical effect on seasonal mean temperature is 0.5-3 °C, which is comparable to greenhouse gas forcing. The effect on seasonal extreme temperature (minimum and maximum) is larger than on mean temperature. Increased (decreased) mean temperature leads to an even larger increase (decrease) in maximum (minimum) temperature. The effect on precipitation is found to be small. Two additional simulations where vegetation is only changed in half of the domain were also performed. These simulations show that the climatic effects from changed vegetation are local. The results imply that vegetation changes have had and will have a significant impact on local climate, therefore these effects from vegetation change should be taken into account when simulating past, present and future climate. The results also imply that vegetation changes could be used to mitigate local climate change.

Strandberg, Gustav

Abstract [en]

Studies in paleoclimate are important because they give us knowledge about how the climate system works and puts the current climate change in necessary perspective. By studying (pre)historic periods we increase our knowledge not just about these periods, but also about the processes that are important for climatic variations and changes. This thesis deals mainly with the interaction between climate and vegetation. Vegetation changes can affect climate in many different ways. These effects can be divided into two main categories: biogeochemical and biogeophysical processes. This thesis studies the biogeophysical effects of vegetation changes on climate in climate models. Climate models are a necessary tool for investigating how climate responds to changes in the climate system, as well as for making predictions of future climate. The biogeophysical processes are strongly related to characteristics of the land surface. Vegetation changes alter the land surface’s albedo (ability to reflect incoming solar radiation), roughness and evapotranspiration (the sum of evaporation and tran-spiration), which in turn affects the energy fluxes between the land surface and the atmosphere and thereby the climate. It is not, however, evident in what way; denser vegetation (e.g. forest instead of grassland) gives decreased albedo, which results in higher temperature, but also increased evapotranspiration, which contrastingly results in lower temperature. Vegetation changes are in this thesis studied in four different (pre)historic periods: two very cold periods with no human influence (c. 44,000 and 21,000 years ago), one warm period with minor human influence (c. 6,000 years ago) and a cold period with substantial human influence (c. 200 years ago). In addition to that the present climate is studied. The combination of these periods gives an estimate of the effect of both natural and anthropogenic vegetation on climate in different climatic contexts. The results show that vegetation changes can change temperature with 1–3 °C depending on season and region. The response is not the same everywhere, but depends on local properties of the land surface. During the winter half of the year, the albedo effect is usually most important as the difference in albedo between forest and open land is very large. During the summer half of the year the evapotranspiration effect is usually most important as differences in albedo between different vegetation types are smaller. A prerequisite for differences in evapotranspiration is that there is sufficient amount of water available. In dry regions, evapotranspiration does not change much with changes in vegetation, which means that the albedo effect will dominate also in summer. The conclusion of these studies is that vegetation changes can have a considerable effect on climate, comparable to the effect of increasing amounts of greenhouse gases in scenarios of future climate. Thus, it is important to have an appropriate description of the vegetation in studies of past, present and future climate. This means that vegetation has the potential to work as a feedback mechanism to natural climatic variations, but also that man can alter climate by altering the vegetation. It also means that mankind may have influenced climate before we started to use fossil fuel. Consequently, vegetation changes can be used as a means to mitigate climate change locally.